Fix hypermedia index: return empty string instead of nil/404

Same pattern as tools — passthrough returning nil fell through to
parent geography page. Now returns empty string so layout renders
with correct nav context.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sx/page-functions.sx

@@ -38,7 +38,7 @@
 
 (define etc (fn (content) (if (nil? content) (quote (<>)) content)))
 
-(define hypermedia (fn (content) (if (nil? content) nil content)))
+(define hypermedia (fn (content) (or content "")))
 
 (define
   reactive